1. Field of the Invention
The present invention relates to an optical beam scanning apparatus which can be used, for example, in office automation equipment, such as a laser printer or a laser facsimile, or a laser plotter which is adapted to drawing, for example, a pattern of a semiconductor circuit, or a laser testing device which is adapted to checking for breakage of a wiring of a printed circuit board, etc.
2. Description of Related Art
Such a known optical beam scanning apparatus is usually comprised of a rotatable polygonal mirror and an f-.theta. lens in combination.
It is also known to use a hologram instead of the combination of the polygonal mirror and the f-.theta. lens to reduce the number of components and ease the required mechanical precision. Generally speaking, a hologram which is made of a thin film of a few .mu.m is lighter and smaller than an optical lens, but has a wavefront conversion function or an aspherical lens function equivalent to or better than that of the optical lens.
In the optical beam scanning apparatus using the rotatable polygonal mirror and the f-.theta. lens to linearly scan an object with a laser beam, however, it is necessary not only to very precisely machine the rotatable polygonal mirror, but also to provide many lens groups including the f-.theta. lens which serves also as an optical system to correct the inclination of the lens surfaces, thus resulting in an increase in manufacturing cost.
For instance, Japanese Unexamined Patent Publication (Kokai) Nos. 62-234118 and 1-244420 disclose an optical beam scanner using a hologram disc which is adapted to effect a precise linear scanning of a laser beam whose aberration is fully corrected and which has a high resolution. Such a known optical beam scanner using a hologram disc satisfies the requirements of precision needed for a scanning optical system of a laser printer to obtain high quality printing.
There has recently been a need for realization of less expensive optical beam scanners having higher resolution. As is well known, the hologram can be inexpensively mass-produced by copying the same and has a very high resolution, and accordingly, the hologram can satisfactorily respond to this need.
Japanese Patent Application Serial No. 3-62961 assigned to the same assignee as the present application discloses an optical beam scanner having a high resolution. In this related application and as shown in FIG. 8 herein, the optical beam scanner is comprised of one laser source 50 (e.g., LD), one rotatable hologram disc 51, and one stationary hologram plate 52. The stationary hologram plate 52 is located between the rotatable hologram disc 51 and a surface 53 (e.g., photosensitive drum) to be scanned (referred to as a scanning surface). The hologram disc 51 has an annular hologram area which is uniformly divided into a plurality of identical hologram facets. The laser beam incident upon the rotatable hologram disc 51 from the LD 50 through a collimating lens 55 is converted to a scanning laser beam by the hologram facets 51a and is then made incident upon the stationary hologram plate 52, so that the scanning beams which are reduced are emitted therefrom. The reduced scanning beams are converged onto the scanning surface 53 to be imaged at an extremely high resolution. Numeral 90 designates a synchronizing photodetector which receives a beam from the hologram disc 51 through the hologram plate 52 to issue a printing start signal every time lines to be printed change.
It is sometimes required that the resolution is switched within one scanning beam. For instance, in a laser printer, it is necessary to switch the resolution of one scanning beam to print figures and/or letters, etc., at different dpi's (dots per inch). Looking at FIG. 2, there are two different printing information areas of 480 dpi and 600 dpi contained in one page of papers to be printed.
In theory, it is possible to realize two different resolutions by providing two light sources having different resolutions in an optical beam scanner employing a rotatable polygonal mirror, or by providing two hologram discs in an optical beam scanner employing a hologram, respectively. This, however, requires two beam scanning optical systems and independent adjustments thereof to prevent a possible deviation of the scanning beams of the respective beam scanning optical systems, thus resulting in a complex and expensive scanner. In addition, it is next to impossible to independently adjust the two beam scanning optical systems.
Furthermore, in a known optical beam scanner, as mentioned above, it is possible to switch the resolution only between different pages. Namely, it is only possible to print an image, for example, at 480 dpi on one page and another image, for example, at 600 dpi on another page.
Accordingly, it is still impossible to switch the resolution on the same page. Namely, images cannot be printed at different dpi's on one page.
The primary object of the present invention is to provide a simple optical beam scanner having a high resolution, wherein the resolution of the scanning beam can be easily switched, even within the same page, without a complex mechanism.